By Josh Cosford, Contributing Editor
Hydraulic hoses are constructed from a combination of soft polymers and wire reinforcements, most commonly made from high-tensile steel. Although the choice of hydraulic fluid plays little role in the degradation of steel, the materials chosen for the inner tube and outer cover are affected by the fluid you run through your hose assemblies.
The primary consideration for fluid and hose compatibility with any given hydraulic fluid is the tube construction. The five most popular material choices for inner tubes are nitrile (NBR or Buna), neoprene, EPDM, Viton and thermoplastics, with nitrile being the most common. Nitrile is a versatile material great for use with most hydraulic oils, as well as high water-based fluids when using the hydrogenated nitrile variant.
With regular nitrile (NBR is a contraction of Nitrile Butadiene Rubber), the cycles of heat and oxidation can lead to swelling, softening and cracking over time when you run water or water glycol. Over time, your hose will experience reduced mechanical strength and eventual failure. EPDM is your best bet for water-based hydraulic fluids. You will often find pump suction hose one and the same as “truck hose,” which is often constructed of EPDM and also rated for vacuum service.
EPDM (Ethylene Propylene Diene Monomer) is the go-to material of choice for applications that require a water-based fluid. Such fluids could be ethylene-glycol mixtures or water-oil emulsions and are a top choice for places like a steel mill. Because of the high heat (both ambient and localized), any hose leaks with these fluids won’t turn your hydraulic plumbing into a flame thrower. I’m not going to provide the explication of PTFE, but just know we also call it Teflon, and it makes a superior choice for water-based fluids in extreme heat applications, although it does make some flexibility sacrifices.
Viton (FKM) is a type of fluoroelastomer that also provides superior protection against aggressive fluids such as polyol ester and phosphate ester, although their general use of those fluids is starting to wane. Phosphate ester is best known for its use as a fire-resistant fluid for aerospace, but it made its way into steel mills as well. Both types of esters are fully synthetic and designed to avoid quick ignition when exposed to flame and extreme heat. In such cases, alternate hose materials such as nitrile and EPDM will degrade and fail, so avoid using them for fire-resistant fluids of all types.
Of course, you must also consider the outer cover when discussing compatibility, especially when you factor in the ambient conditions of your hydraulic hose. Industrial environments might have aggressive chemicals or solvents that could damage or degrade a standard neoprene cover. Again, this is where Viton comes into play, which is resilient in the face of everything except water, strangely enough, so mobile applications with chemical exposure may require more exotic solutions, such as PTFE with stainless steel braids.
By respecting the traditional recommendations for chemical compatibility, you can be sure to select the appropriate hose material to create the most effective and reliable hose assemblies. Although off-the-shelf hose provides a solution to 90% of the applications you will see, make sure you know all the details of the machine and environment to prepare for all scenarios.
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